(x+2y+Z)^2-2(2xy+xz+2yz)=х^2+2ху+хz+2ху+4у^2+2уz+хz+2уz+z^2-4ху-2хz-4уz=
=х^2+4у^2+z^2+(2ху+2ху-4ху)+(хz+хz-2хz)+(2уz+2уz-4уz)=
=х^2+4у^2+z^2+0+0+0=х^2+4у^2+z^2
(x+2y+Z)^2-2(2xy+xz+2yz)=х^2+2ху+хz+2ху+4у^2+2уz+хz+2уz+z^2-4ху-2хz-4уz=
=х^2+4у^2+z^2+(2ху+2ху-4ху)+(хz+хz-2хz)+(2уz+2уz-4уz)=
=х^2+4у^2+z^2+0+0+0=х^2+4у^2+z^2